The building defects are always of great concern for construction industry as they cause delays before handing over and increase maintenance costs during occupancy of the buildings. Due to the rapid development and population growth in Gaza Strip, many residential buildings and housing projects are being constructed in a short space of time, especially after Gaza war in 2014. Therefore, many errors and defects are expected to arise during the construction stage. The aim of this paper is to identify and rank the factors affecting the defects occurrence in the construction stage of the residential buildings in Gaza Strip. A survey was conducted in 134 randomly selected engineers working for engineering offices as design engineers, supervisors and project managers in Gaza Strip. The study revealed five most effective main factors during the construction stage, namely construction materials, factors due to inspections, factors due to construction equipment, factors due to constructions and factors due to construction management, respectively. The most important factors in each group were poor soil compaction, exceeding the allowable limits of verticality of the structural elements, insufficient reinforcement concrete cover, owner's negligence of inspections, absence of engineer in most of construction phases, using expired material, material or component failure, using corroded or secondhand reinforcement steel, using materials not of acceptable quality and not conforming to the specifications or design and lack of required equipment. This research is the first attempt at identifying factors affecting defect occurrence in the construction stage of residential buildings of the developing countries. The findings will be useful for the construction firms and regulatory bodies to deal with the critical factors affecting defect occurrence and device such policies that improve the design and construction of residential buildings.